The present invention concerns a process to produce alkyl 6-aminocaproate and/or caprolactam.
Commercially, caprolactam is made by a process using cyclohexane as the starting material. Caprolactam is then polymerized to produce nylon-6. For cost reasons, it would be desirable to produce caprolactam with butadiene, a four carbon starting material, rather than the six carbon cyclohexane starting material currently used in commercial processes.
It is known that butadiene can be reacted with HCN to produce 3-pentenenitrile (3PN). One process for converting 3PN to caprolactam involves converting 3PN to adiponitrile (ADN). ADN is then partially hydrogenated to 6-aminocapronitrile, which is then converted to caprolactam by hydrolysis followed by cyclization. See for example, U.S. Pat. No. 6,069,246. The partial hydrogenation reaction produces a significant amount of hexamethylenediamine (HMD).
A second process for converting 3PN to caprolactam involves reductive amination of 5-formylvaleronitrile that is derived by hydroformylation of 3-pentenenitrile. The reductively aminated product is then subjected to hydrolysis and cyclized. U.S. Pat. No. 6,048,997 discloses a process in which a mixture containing 2-, 3-, and 4-pentenenitrile is reacted with carbon monoxide and hydrogen in the presence of a catalyst containing at least one Group VIII metal to produce a mixture comprising 3-, 4-, and 5-formylvaleronitrile. U.S. Pat. No. 5,986,126 teaches that 5-formylvaleronitrile is unstable and that the separation of 5-formylvaleronitrile from the branched 3- and 4-formylvaleronitriles is impractical because of yield losses that are suffered in distillation. To avoid this problem, U.S. Pat. No. 5,986,126 teaches that the separation of the linear product from the branched isomers is possible downstream after reductive amination of the formylvaleronitriles to produce aminonitriles (such as 6-aminocapronitrile) and diamines. In this second process, a significant amount of HMD is produced.
Both of the two 3PN-based processes described above produce significant amounts of HMD. It is not always desired to have HMD as a co-product in a commercial caprolactam operation. Thus, there is a need for a process that converts butadiene to caprolactam without the production of significant amounts of HMD. The present invention provides such a process.
The present invention is a 3PN-based process for making alkyl 6-aminocaproate that does not produce significant amounts of HMD. The present invention accomplishes this by a process comprising:
(a) reacting 3-pentenenitrile with carbon monoxide and hydrogen in the presence of a hydroformylation catalyst comprising a Group VIII metal to produce a first reaction product which comprises 3-, 4-, and 5-formylvaleronitrile (FVN);
(b) isolating from the first reaction product a FVN mixture consists essentially of 3-, 4-, and 5-formylvaleronitrile;
(c) reacting the FVN mixture to produce a second reaction product which comprises alkyl 3-, 4-, and 5-cyanovalerate by either:
(i) contacting the FVN mixture with an alcohol, a molecular oxygen-containing gas, and a palladium-containing catalyst for a time sufficient to oxidize the FVN mixture to produce the second reaction product, or
(ii) oxidizing the FVN mixture in the presence of a molecular oxygen-containing gas for a time sufficient to oxidize the FVN mixture to produce an oxidation product comprising 3-, 4-, and 5-cyanovaleric acid, and reacting the oxidation product with an alcohol to produce the second reaction product;
(d) isolating the alkyl 5-cyanovalerate by distillation, and
(e) reacting alkyl 5-cyanovalerate with hydrogen in the presence of a hydrogenation catalyst to produce a third reaction product which comprises alkyl 6-aminocaproate, the alkyl group of which contains the same number of carbon atoms as the alcohol.
The present invention concerns a process for the production of alkyl 6-aminocaproate. Suitable alkyl groups are C1 to C12 linear or branched alkyl groups. Preferably the alkyl group is methyl or ethyl. More preferably, the alkyl group is methyl.
3-Pentenenitrile (3PN) is produced commercially as an intermediate in the production of adiponitrile. The synthesis of 3PN is well known in the art. See for example, U.S. Pat. Nos. 3,496,215 and 5,821,378, the disclosures of which are incorporated herein by reference.
The hydroformylation of 3-pentenenitrile (i.e., the reaction of 3-pentenenitrile with carbon monoxide and hydrogen) to produce a reaction product which comprises 3-, 4-, and 5-formylvaleronitrile (FVN) is carried out in the presence of a catalyst comprising a Group VIII element. The hydroformylation reaction temperature can vary from room temperature to about 200xc2x0 C., preferably between 50 and 150xc2x0 C. The pressure is preferably between 0.15 and 10 MPa and more preferably 0.2 to 5 MPa.
Preferred catalysts are rhodium compounds. Examples of suitable compounds include Rh(CO) 2 (DPM), [DPM=txe2x80x94C4H9xe2x80x94COCHCOxe2x80x94txe2x80x94C4H9]; Rh(CO)2(acac), [acac=acetylacetonate]; Rh2O3; Rh4(CO)12; Rh6(CO)16; [Rh(OAc)2]2, [OAc=acetate]; and Rh(ethylhexanoate)2. Preferably, the catalyst is Rh(CO)2(acac), Rh(CO)2(DPM), or [Rh(OAc)2]2.
These catalysts can be used in combination with phosphorous-containing ligands such as monodentate or bidentate phosphine, phosphonites, phosphinites, or phosphite compounds. Examples of such ligands include triarylphosphites, such as triphenylphosphite; triarylphosphines, such as triphenylphosphine; and bis(diarylphosphino)alkanes, such as diphenylphosphinoethane. In addition, polydentate phosphite compounds may be used as ligands. An example of these includes compounds having a structural formula as follows: 
where R1 and R2 are the same or different mono-valent aryl groups, X is an n-valent organic bridging group, and n is an integer between 2 and 6. R1 and R2 may be substituted. Such ligands are described, for example, in U.S. Pat. No. 5,710,344, the disclosure of which is incorporated herein by reference.
The mole ratio of 3-pentenenitrile to catalyst is generally 100:1 to 100,000:1, preferably 500:1 to 10,000:1. The mole ratio of ligand to rhodium is typically between 0.5:1 and 10:1.
The mole ratio of hydrogen to carbon monoxide for hydroformylation reactions is typically in the range of 100:1 to 1:10, preferably in the range of 4.0:1 to 0.5:1. Inert gases may also be present in the hydrogen and carbon monoxide feed stocks.
The hydroformylation reaction may be performed in the presence of a solvent. Suitable solvents include inert solvents or a solvent consisting of the hydroformylation products themselves. Suitable inert solvents include aromatic hydrocarbons, aliphatic hydrocarbons, nitriles, ethers, amides and urea derivatives, saturated hydrocarbons, and ketones. Some examples of suitable solvents include toluene, cyclohexane, benzene, xylene, Texanol(copyright) (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), diphenylether, tetrahydrofuran, cyclohexanone, benzonitrile, N-methylpyrrolidinone, and N,N-dimethylethylurea.
The hydroformylation reaction can be performed in a continuous or batch mode. The reaction can be performed in a variety of reactors, such as bubble column reactors, continuously stirred tank reactors, trickle bed reactors, and liquid-overflow reactors. Unreacted hydrogen, carbon monoxide, 3-pentenenitrile, and any solvent may be recovered and recycled to the hydroformylation reactor.
The hydroformylation reaction product comprises 3-, 4-, and 5-formylvaleronitriles, as well as unconverted 2-, 3-, and 4-pentenenitrile, catalyst, and high boilers. The separation of the FVN mixture from the catalyst and high boilers can be effected by utilizing thermally gentle evaporation techniques, known to those skilled in the art. Such techniques include the use of single stage flash evaporators, such as rolling-film evaporators, falling-film evaporators, or wiped-film evaporators. High boilers and catalyst separated from the FVN mixture can be recycled back to the hydroformylation reactor.
To avoid the decomposition of the catalyst and FVN mixture, a short contact time during flash evaporation is generally preferred. The contact time can vary between 1 second and 1 hour and preferably is between 1 and 5 minutes. The flash evaporation is carried out under commercially viable operating conditions. The temperature should be in the range of 75 to 200xc2x0 C. The preferred range is 100 to 130xc2x0 C. The pressure can vary from 13.3 to 1333 Pa, preferably 66.6 to 666.5 Pa.
Alkyl 5-cyanovalerate can be made by oxidative esterification of 5-formylvaleronitrile (5-FVN or 5FVN). In this process, 5-FVN is exposed to an alcohol and an oxygen containing gas in the presence of a palladium-containing catalyst.
The FVN mixture is contacted with a molecular oxygen-containing gas for a time sufficient to oxidize the FVN mixture to produce a reaction product containing alkyl 3-, 4-, and 5-cyanovalerate (mixed ACV). Preferably, the oxidation is performed at a pressure of 100 to 5000 psig (0.7 to 34.5 MPa) in the presence of air. More preferably, the pressure is 500 to 2000 psig (3.4 to 13.8 MPa). Such reaction conditions give a high conversion rate. The reaction may be run as a continuous process.
The oxidative esterification step of the present invention can be performed at a temperature of from about 20xc2x0 C. to about 120xc2x0 C. Preferably, the temperature is in the range of about 40xc2x0 C. to about 80xc2x0 C. Since the oxidative esterification is exothermic, operating a commercial reactor at about 50xc2x0 C., and above, is preferred as heat removal and associated cost become economic considerations. It is preferable to choose a temperature that allows the use of normal, low-cost cooling water.
The alcohol used in the oxidative esterification may be any alcohol that does not interfere with subsequent reaction steps. Preferably, the alcohol is a linear or branched C1 to C12 alcohol. More preferably, the alcohol is methanol or ethanol. The reaction can advantageously be run in the presence of a stochiometric excess of alcohol.
Suitable solvents for the oxidative esterification can be selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, alcohols and esters. Of particular importance are alcohols that can also function as the solvent for the oxidative esterification. The alcohol to aldehyde ratio ranges from 1:1 to 50:1.
The palladium-containing catalyst may be any palladium catalyst capable of catalyzing oxidative esterification of 5-formylvaleronitrile. Preferably, the catalyst is a heterogeneous palladium-based catalyst as described in European Patent Application 199530. Examples of suitable catalysts include Pd4TeZnPb, Pd4TeZnPbBi, and Pd4TeZn.
The actual method of commercially implementing the oxidative esterification process according to the present invention can be by any air oxidation method, as generally known in the art, including, by way of example, but not by limitation, batch reactor with or without stirring, continuous reactor with plug flow or back-mixing, counter current reactor and the like.
Alkyl 5-cyanovalerate can be separated from the reaction mixture by fractional distillation. A stage of evaporation would be used to separate the much lower boiling alcohol (methanol is preferred) from the bulk of the product from oxidative esterification. This separation would be accomplished at a pressure of 1.3xc3x9710xe2x88x923 MPa to 6.5xc3x9710xe2x88x922 MPa preferably, 6.5xc3x9710xe2x88x923 MPa to 3.5xc3x9710xe2x88x922 MPa. Evaporator temperature would be set to permit near complete removal of methanol, 80 to 200xc2x0 C., more preferably 100 to 150xc2x0 C. The methanol rich distillate stream would be recycled to the oxidative esterification. The mixed ACV product, thermally stable compounds now free of methanol, would then be refined in a traditional staged distillation column.
The mixed ACV product is a mixture comprising alkyl-5-cyanovalerate and its branched isomers, alkyl-4-cyanovalerate, and alkyl-3-cyanovalerate. The mixture may be distilled at a pressure of 1.3xc3x9710xe2x88x923 MPa to 6.5xc3x9710xe2x88x922 MPa preferably, 6.5xc3x9710xe2x88x923 MPa to 3.5xc3x9710xe2x88x922 MPa. In one possible configuration of the process, the mixed ACV stream is fed to the middle section of a distillation column. The branched materials are taken overhead and can be burned or converted to specialty chemicals. The refined linear material exits the column reboiler and can be fed directly to hydrogenation. Typically, the column temperatures are between 100 and 250xc2x0 C., preferably 140 to 200xc2x0 C.
As an alternative to oxidative esterification, the FVN mixture can be oxidized in the absence of an alcohol and then esterified. 5-Cyanovaleric acid can be made by oxidation of 5-formylvaleronitrile by a process similar to that taught in U.S. Pat. No. 5,840,959, where methyl-5-formylvalerate is oxidized to produce monomethyladipate.
The FVN mixture is contacted with a molecular oxygen-containing gas for a time sufficient to oxidize the FVN mixture to produce a reaction product containing 3-, 4-, and 5-cyanovaleric acid. FVN can be oxidized with or without a catalyst and at atmospheric or elevated pressure. U.S. Pat. Nos. 4,537,987 and 4,931,590 teach that alkali metal oxides (such as potassium hydroxide or sodium hydroxide in amounts of 0.001 to 0.5% by weight) and metal salts of cobalt or manganese (such as cobalt acetate or manganese acetate in amounts of 0.0001 to 0.1% by weight) can be used to accelerate the oxidation reaction. While these catalysts can be used with the present invention, it is preferred to run the oxidation reaction in the absence of such catalysts.
Preferably, the oxidation is performed at elevated pressure in the presence of air. Such reaction conditions give a high conversion rate. The reaction may be run as a continuous process.
To obtain high conversion and selectivity, a pressure above atmospheric pressure (about 1 MPa) and preferably above 10 bars (1 MPa) of air is required. More preferably, the total pressure when using air should be about 20 bars (2 MPa) or higher. While higher pressures, e.g., 40 to 65 bars (4 to 6.5 MPa), may improve reactivity, they can necessitate higher equipment cost. Pressures of from about 20 to 40 bars (2 to 4 MPa) air represent a realistic and commercially acceptable range.
The oxidation step of the present invention can be performed at a temperature of from about 20xc2x0 C. to as high as about 120xc2x0 C. Preferably, the temperature is in the range of about 40xc2x0 C. to about 80xc2x0 C. Since the oxidation is exothermic, operating a commercial reactor at about 50xc2x0 C., and above, is preferred as heat removal and associated cost become economic considerations. It is preferable to choose a temperature that allows the use of normal, low-cost cooling water.
The actual method of commercially implementing the oxidation process according to the present invention can be by any non-catalytic, heterophase, air oxidation method, as generally known in the art, including, by way of example, but not by limitation, batch reactor with or without stirring, continuous reactor with plug flow or back-mixing, counter current reactor and the like. U.S. Pat. No. 5,840,959 teaches that for oxidation of alkyl 5-formylvalerate, realistic heat removal considerations cause the preferred method of reactor operation to be at less than optimum conversion. However, due to the high boiling point of the 3-, 4-, and 5-cyanovaleric acids in the present invention, it is preferred to run the oxidation reaction at the highest possible conversion and selectivity. Such an operation avoids the need to run a recycle loop with its associated distillation requirements.
Following the oxidation, the product can be esterified. Organic esters can be made by reaction of the appropriate carboxylic acid and alcohol in the presence of a homogeneous or heterogeneous catalyst. One of the most common homogeneous catalysts is sulfuric acid, and the most common heterogeneous catalysts are ion-exchange resins. Heterogeneous acidic catalysts have proved to be useful in many applications because of their activity, selectivity, reusability, non-corrosivity and virtual absence of effluent treatment which is associated with the homogeneous catalysts. In the present invention the esterification process is conducted in the presence of primary alcohols having from 1 to 4 carbon atoms. The temperature required for operation ranges from 25 to 150xc2x0 C. with the preferred range being 70 to 120xc2x0 C. In order to achieve high yields of esters the reaction between the acid and the ester is conducted in the presence of excess alcohol. The preferred catalysts used are the sulfonic type cation exchange resins, having a macroreticular structure. As the name implies, these are used in their acid form. These catalysts, their properties and method of preparation are taught in U.S. Pat. No. 3,037,052. The catalysts are available commercially and are sold under the trademark Amberlyst-15 (Rohm and Haas Company). The reaction is carried out in a non-aqueous system, the reactants and catalyst being substantially anhydrous. The reaction can be carried out either in batch or continuous manner.
In the present invention, the mixture of 3-, 4-, and 5-cyanovaleric acids is reacted with a linear or branched C1 to C12 alcohol to produce a mixture of alkyl 3-, 4-, and 5-cyanovalerate. More preferably, the alcohol is methanol or ethanol. Alkyl 5-cyanovalerate is isolated from the reaction mixture by fractional distillation as described in the preceding discussion of oxidative esterification.
Hydrogenation of the nitrile group to produce alkyl 6-aminocaproate from alkyl 5-cyanovalerate, can be accomplished in the presence of a metal catalyst, and optionally in a liquid solvent. Suitable metal catalysts can be of many types. The catalyst is used in an amount effective to catalyze the reaction. For example, sponge metal catalysts, homogeneous catalysts, and reduced metal oxide and mixed metal oxide catalysts may be used. Supported metal catalysts may be also used. Suitable active metals include iron, ruthenium, rhodium, iridium, palladium, cobalt, nickel, chromium, osmium, and platinum.
Sponge metals are one class of catalysts useful for the present invention. A sponge metal has an extended xe2x80x9cskeletonxe2x80x9d or xe2x80x9csponge-likexe2x80x9d structure of metal, with dissolved aluminum, and optionally contains promoters. The sponge metals may also contain surface hydrous oxides, absorbed hydrous radicals, and hydrogen bubbles in pores. Sponge metal catalysts can be made by the process described in U.S. Pat. No. 1,628,190, the disclosure of which is incorporated herein by reference.
Preferred sponge metals include nickel, cobalt, iron, ruthenium, rhodium, iridium, palladium, and platinum. Sponge nickel or sponge cobalt are particularly suitable as catalysts. The sponge metal may be promoted by one or more promoters selected from the group consisting of Group IA (lithium, sodium, and potassium), IB (copper, silver, and gold), IVB (titanium and zirconium), VB (vanadium), VIB (chromium, molybdenum, and tungsten), VIIB (manganese, rhenium), and VIII (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum) metals. The promoter can be used in an amount useful to give desired results. For example, the amount of promoter may be any amount less than 50% by weight of the sponge metal, preferably 0 to 10% by weight, more preferably 1 to 5% by weight.
Sponge nickel catalysts contain mainly nickel and aluminum. The aluminum is typically in the form of metallic aluminum, aluminum oxides, and/or aluminum hydroxides. Small amounts of other metals may also be present either in their elemental or chemically bonded form, such as iron and/or chromium, and may be added to the sponge nickel to increase activity and selectivity for the hydrogenation of certain groups of compounds. It is particularly preferred to use chromium and/or iron promoted sponge nickel as a catalyst.
Sponge cobalt catalysts also contain aluminum and may contain promoters. Preferred promoters are nickel and chromium, for example in amounts of about 2% by weight based on the weight of the catalyst.
Examples of suitable sponge metal catalysts include Degussa BLM 112W, W. R. Grace Raney(copyright) 2400, Activated Metals A-4000(trademark), and W. R. Grace Raney(copyright) 2724.
Supported metal hydrogenation catalysts are another kind of useful catalysts for the present invention. Such catalysts consist of a metal catalyst on a solid support. Any such catalyst may be used in catalytically effective amounts. Preferred metals in the supported metal catalyst include ruthenium, nickel, cobalt, iron, rhodium, iridium, palladium, and platinum. Ruthenium is especially preferred. More than one metal may be used. Any solid support that does not interfere with the reaction can be used. Preferred solid supports include titanium dioxide, porous aluminum oxide, silicon dioxide, aluminum silicate, lanthanum oxide, zirconium dioxide, activated charcoal, aluminum silicate, silicon dioxide, lanthanum oxide, magnesium oxide, zinc oxide, and zeolites.
Particularly preferred solid supports are titanium dioxide, porous aluminum oxide, silicon dioxide, zirconium dioxide, and activated charcoal. Especially useful supported metal catalysts are supported ruthenium catalysts, for example, ruthenium on titanium dioxide. Also, it is acceptable to use a mixture of more than one support and/or more than one catalyst element.
Any method of placing the metal on the support may be used. Several methods are known in the art. One method uses vapor deposition of the metal onto the support. Another method uses a flame spray technique to apply the metal to the support. Another method applies a solution of the metal salt or metal oxide to the support. This step is followed by drying of the support and then reducing the salt or oxide. Another method applies a metal salt that can easily be thermally decomposed to the support. Suitable metal salts include carbonyl or hydride complexes of one or more of iron, nickel, cobalt, ruthenium, rhodium, palladium, osmium, iridium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, copper, silver, and gold.
The metal is typically applied to the solid support at 0.1 to 90 percent by weight relative to the total weight of the supported catalyst. Preferably, the metal is at 0.5 to 50% by weight, more preferably 2 to 25% by weight.
Homogeneous catalysts are another useful type of metal catalyst for the present invention. Homogeneous catalysts are soluble metal compounds incorporating one or a combination of metals such as rhodium, ruthenium, cobalt, nickel, iron, palladium, or platinum, and a hydrocarbon containing ligand which may also contain an atom bonded to the metal atom such as phosphorus, nitrogen, oxygen, carbon, and sulfur.
Another type of useful hydrogenation catalyst is derived from the reduction of at least one metal oxide, a mixture of metal oxides, or a mixture of metal oxide, hydroxide and/or carbonate. Such catalysts have similar structures to sponge metal catalysts in their extended xe2x80x9cskeletonxe2x80x9d metallic structure. However, they typically would not contain dissolved aluminum or silicon. Such catalysts can be prepared by the reduction of bulk metal oxides such as iron oxide or cobalt oxide. Alternately, the bulk metal oxide precursor may be prepared as a mixture of metal oxides including one or more of the oxides of iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, chromium, molybdenum, tungsten, and manganese. In addition, metal hydroxides or metal carbonates may be included in the metal oxide mixture. See International Patent Application WO 98/04515 and U.S. Pat. No. 6,005,145, the latter being incorporated herein by reference.
The hydrogenation reaction is normally performed at a pressure of 100 to 5000 psi (0.69 to 34.5 MPa), preferably 300 to 1500 psi (2.1 to 10.3 MPa), and more preferably 500 to 1000 psi (3.4 to 6.9 MPa). The hydrogen partial pressure is typically 50 to 4000 psi (0.34 to 27.6 MPa), preferably 100 to 1000 psi (0.69 to 6.9 MPa), and more preferably 250 to 750 psi (1.7 to 5.2 MPa). The molar ratio of hydrogen to alkyl 5-cyanovalerate is typically 2:1 to 200:1, more preferably, 2:1 to 100:1.
The hydrogenation reaction temperature is 40 to 220xc2x0 C., preferably 70 to 150xc2x0 C., more preferably 80 to 120xc2x0 C.
The reaction is preferably carried out in the absence of air.
The hydrogenation reaction may optionally be performed in the presence of a solvent. Any solvent that does not interfere with the reaction may be used and can be used in an amount to increase the yield of the reaction and/or to remove heat from the reaction. Suitable solvents include water, alcohols, esters, hydrocarbons, tetrahydrofuran (THF), dioxane, ammonia, and ammonium hydroxide. Preferred solvents are ammonia, methanol, water, and mixtures of these solvents. Typically when a solvent is used, the mole ratio of solvent to alkyl 5-cyanovalerate is 1:1 to 100:1, preferably 5:1 to 40:1, more preferably 10:1 to 20:1.
Hydrogenation reactions may be performed in any suitable type of reactor. Suitable reactors include a fixed bed reactor and slurry reactor. A fixed bed reactor has an advantage of easy separation of the reactants and products from the catalyst. Slurry reactors include batch, a continuously stirred tank reactor, and a bubble column reactor. In slurry reactors, the catalyst may be removed from the reaction mixture by filtration or centrifugal action.
The amount of hydrogenation catalyst used will depend on the type of reactor used. For slurry reactors, the catalyst will make up 0.1 to about 30% by weight of the reactor contents. Preferably, the amount of catalyst will be 1 to 15% by weight, more preferably 5 to 10% by weight.
For a fixed bed reactor, the weight hourly space velocity will typically fall in the range of 0.05 to 100 hrxe2x88x921, preferably 0.1 to 10 hrxe2x88x921, more preferably 1.0 to 5.0 hrxe2x88x921.
U.S. Pat. No. 4,730,040, incorporate herein by reference, describes a process where alkyl 6-aminocaproate can be hydrolyzed to 6-aminocaproic acid which can be cyclized to xcex5-caprolactam at elevated temperatures (specifically 150 to 370xc2x0 C.). U.S. Pat. No. 5,877,314, incorporated herein by reference, discloses a process where an alkyl 6-aminocaproate is converted to caprolactam and caprolactam precursors by reaction of alkyl 6-aminocaproate with hydrogen and excess ammonia in the presence of a ruthenium catalyst. The alcohol, typically methanol, is removed from the reaction mixture prior to cyclization.